This invention relates to a process for the manufacture of copolymers of formaldehyde and diphenyl oxide having dependent unsaturation derived from the concurrent reaction with olefinically unsaturated mono- or dicarboxylic acids.
In the manufacturing process for copolymers of formaldehyde (CHO), diphenyl oxide (DPO) and olefinically unsaturated mono- or dicarboxylic acids, taught by U.S. Pat. No. 3,914,194, the reaction requires the presence of a strong inorganic or organic catalyst having a pKa of less than 0.5 in water.
The reaction of formaldehyde and the acid produces the methylol cation +CH2OH. Attack of this cation on the phenyl rings of DPO followed by condensation with further DPO leads to the oligomeric backbone of the copolymer. Terminal +CH2OH groups then react with the olefinically unsaturated mono or dicarboxylic acid to form the corresponding ester end groups.
A key specification for these copolymer products is the viscosity of the neat resin, and a key requirement is that the viscosity level be consistent from batch to batch. A problem with using a strong acid as catalyst in the manufacturing process is that viscosity of the neat resin can vary from less than 10,000 mPa.s to greater than 100,000 mPa.s even when the processing parameters are held within a very narrow range from batch to batch.
Another key specification in the manufacture of these copolymers is that residual levels of starting materials in the final product be sufficiently low so as not to interfere with the performance of the final product. Production batches made with strong acid catalysts usually result in levels of residual starting materials that are unacceptable.
This creates a need for a manufacturing process for these materials that is not overly sensitive to small changes in processing parameters so that viscosity and residual starting material amounts can be manipulated to fall within required specifications, and to fall consistently within those specifications from batch to batch.
This invention is a process for the manufacture of a copolymer of formaldehyde or paraformaldehyde, diphenyl oxide, and a mono- or dicarboxylic acid in the presence of an acid catalyst with a pKa within the range of 1.2 to 3. Two consequences of the use of an acid with a pKa within this range are that the product viscosity is less sensitive to reaction conditions compared to the use of acids with lower pKa values, and levels of residual starting materials can be kept to acceptably low levels for commercial use.
The formaldehyde component of this process can be used in all its forms, including solutions in water or aqueous methanol, or paraformaldehyde.
The diphenyl oxide can be unsubstituted, or the aromatic rings can be substituted with alkyl groups of one to ten carbon atoms, or can be halogenated in one or both rings. It is preferred to use diphenyl oxide in solid form and not in an aqueous solution.
Acids with pKa values within the range of 1.2 to 3.0 for use as catalysts include o-aminobenzosulfonic (pKa 2.48), arsenic (pKa 2.25), bromoacetic (pKa 2.69), o-bromobenzoic (pKa 2.84), chloroacetic (pKa 2.85), chlorobenzoic (pKa 2.92), chlorobutyric (pKa 2.86), chloropropionic (pKa 2.83, cyanoacetic (pKa 2.45), cyanobutyric (pKa 2.42), cyanophenoxyacetic (pKa 2.98), dichloroacetic (pKa 1.48), dichloroacetylacetic (pKa 2.11), dihydroxybenzoic (pKa 2.94), dihydroxymalic (pKa 1.92), dinicotinic (pKa 2.80), fluorobenzoic (pKa 2.90), hydroxybenzoic (pKa 2.97), iodobenzoic (pKa 2.85), lysine (pKa 2.15), maleic (pKa 1.83), malonic (pKa 2.83), nitrobenzoic (pKa 2.16), oxalic (pKa 1.23), phosphoric (pKa 2.12), phosphorous (pKa 2.00), phthalic (pKa 2.89), quinolinic (pKa 2.52), tartaric (pKa 2.98), trihydroxybenzoic (pKa 1.68), selenious (pKa 2.46), sulfurous 1.81), tellurous (pKa 2.48).
It is preferred that the acids be used in solid form and not in aqueous solution. Preferred acids are those with a pKa in the range of 1.75 to 2.25. A more preferred acid is phosphorous acid.
Olefinically unsaturated monocarboxylic acids suitable for use in this process will have three to ten carbon atoms and include acrylic, crotonic, angelic, methacrylic, ethacrylic, oleic and linoleic acids. Olefinically unsaturated dicarboxylic acids or anhydrides will have four to twelve carbons atoms and include maleic, fumaric, itaconic, 3-hexene-1,6-dicarboxylic acid, citaconic phenyl maleic, and similar acids.
All the reactants are charged to the reactor and brought up to reaction temperature over a period of one to two hours, and then held at the reaction temperature for a period of time between 18 to 30 hours, preferably 22 to 26 hours. The reaction temperature generally is in a range of 86xc2x0 and greater to 94xc2x0 C. and lower, preferably 88xc2x0 and greater to 92xc2x0 C. and lower, and more preferably 90xc2x0 C. The range for the reaction temperature is generally lower and for the reaction time longer than in prior art processes, which is attributed to the higher pKa values of the acid catalysts. The weaker acid and milder reaction temperature compared to those of prior art processes is believed to make the viscosity of the product less sensitive to minor variations in reaction parameters. The result is product with viscosity consistent from batch to batch.
The stoichiometry of this process is determined relative to DPO. (DPO is the limiting reagent inasmuch as any residual DPO cannot be stripped or washed from the product, and therefore must be fully reacted.) Suitable mole ratios for the reactants per mole of DPO are three to four moles of carboxylic acid and two to three moles formaldehyde.
The ratio to DPO for the acid catalyst will vary depending on the acid chosen, with a higher ratio needed for the weaker of the acids. One skilled in the art can determine the appropriate ratio for the acid catalyst without undue experimentation. Using phosphorous acid, the ratio per mole of DPO will be in the range of 1.8 to 2.2 moles of phosphorous acid.
After the reaction steps are completed, the work-up and isolation of the product may follow any suitable commercial practice. One such commercial practice is as follows. The reaction mixture is cooled, and washed with a mixture of water and methylene chloride. A suitable ratio of water to methylene chloride is in the range of 70:30 to 60:40. Next, the lower product layer is drawn off and washed with water, then drawn off again, and washed with a mixture of methanol and water. A suitable ratio of water to methanol is in the range of 70:30 to 60:40. After this second washing, the product layer is neutralized to pH 7.5-9.0 using ammonium hydroxide. Methanol is added to the neutralized product layer and the aqueous layer is separated from the product layer. The product layer is washed two more times with methanol and water, dried over sieves and then stripped under vacuum with an air sparge and inhibitors (methoxyhydroquinone, hydroquinone, and butylated hydroxytoluene) to 70-90% solids.